Massive Multiple-Input Multiple-Output (MIMO) with the number of antenna elements at a base station being increased by an order of magnitude, from hundreds to thousands, is known as a technology that offers ultra-wideband transmission at a high frequency range. In the case of implementing data stream separation and beam control by digital signal processing (digital precoding) in the baseband domain, load involved in matrix operation increases in proportion to the number of antenna elements. On the other hand, load of the matrix operation is reduced by adopting hybrid beamforming (BF) that combines analog beamforming for controlling beam directions in the radio frequency (RF) domain and digital precoding for separating data streams.
For this reason, the application of hybrid beamforming to base stations with Massive MIMO has been examined. Now let H denote a channel matrix representing the channel state between a base station and a terminal; W denote an analog beamforming matrix representing effects caused by analog beamforming; and P denote a digital precoding matrix representing effects caused by digital precoding. A signal vector r received at the terminal is expressed by the following Equation (1), where d and z denote a transmitted signal vector and a noise vector, respectively.r=HWPd+z   (1)
In order to achieve hybrid beamforming that adequately compensates path-loss, the base station appropriately sets the analog beamforming matrix W and implements a process of estimating HW (explicit CSI (channel state information)) that reflects the analog beamforming and the channel state. In setting the analog beamforming matrix W, the base station finely controls the beam directions and obtains feedback on reception quality from the terminal for each of a large number of beam directions. Then, based on the reception quality fed back thereto, the base station determines the analog beamforming matrix W to be used for data transmission.
After determining the analog beamforming matrix W, the base station transmits reference signals to all terminals using the determined analog beamforming matrix W. Upon receiving the reference signals, the terminals individually generate explicit CSI estimate values based on the received reference signals and then feed the generated estimate values back to the base station. Using the estimate values fed back from all the terminals, the base station calculates the digital precoding matrix P. Then, the base station transmits data using the determined analog beamforming matrix W and the calculated digital precoding matrix P.    Japanese Laid-open Patent Publication No. 2015-164281    International Publication Pamphlet No. WO 2015141066    Japanese National Publication of International Patent Application No. 2015-513257    T. Obara, S. Suyama, J. Shen, and Y. Okumura, “Joint Fixed Beamforming and Eigenmode Precoding for Super High Bit Rate Massive MIMO Systems Using Higher Frequency Bands”, Proceedings of 2014 IEEE 25th Annual International Symposium on Personal, Indoor, and Mobile Radio Communication, Washington D.C., US, September 2014.    Yoshioka, Suyama, Obara, Okuyama, Mashino, and Okumura, “Optimization of Number of Beams for 5G Low-SHF-BAND Massive MIMO with Digital Beamforming in View of Insertion Loss of Pilot Signals”, IEICE (the Institute of Electronics, Information and Communication Engineers) technical report, RCS2016-42.
According to the above-described method, in order to implement hybrid beamforming, transmission and reception of the reference signals and the subsequent feedback between the base station and all the terminals take place over and over for each of the large number of beam directions. A large number of beam directions needed to be considered and the presence of a large number of terminals reduce communication efficiency.